D’ya like dags?
Before all the E&Ms get carried away blogging lets talk about concrete.
Well, the answer if you are an Irish concrete pump operator is no. Despite the Archimedes screw inside a mixer truck a small amount of concrete always remains inside, especially in drum mixers. You can wash the truck out, run water through the drum while it rotates and even pressure-wash the inside. But some concrete residue is always left. And it dries. The next trip a little more is left. And it dries. Over time, this becomes a problem, reducing the volume of the mixer and its capacity.
The process is expedited when a concrete mixer truck driver does not wash out the machine properly between loads and aggregate and chunks of concrete are left to cure in the drum. Once hardened, and a new batch is being spun in the machine at full tilt, the old lumps of concrete can work loose and get fed into your 47m concrete boom pump at 1am. A dag as it turns out is a lump of old, hardened concrete. Pondering this new technical term and having had a quick google, turns out it is defined as ‘a lock of wool matted with dung hanging from the hindquarters of a sheep’.
The pump struggled as it passed through the hopper – we dropped the hopper but the problem persisted. Not knowing how large the ‘dag’ was and hoping it would break up in the line the operator continued pumping concrete only for it to block the 5” to 4” reducer pipe at the bottom of the boom. We retracted the boom, removed the reducer and found the dag! Weighing about 3kg and full of 30mm aggregate – we were pouring a 10mm mix.
The ‘dag’ delayed the pour by an hour. As a site engineer you control the batching times of concrete from the plant and arrival to site. I had to return a near full load (7m3) truck to the batching plant as it got too old for us to use (over two hours). Hot, dry, stiff concrete won’t pump either. Touch the side of the mixer truck and you can feel how old the concrete is. The pump operator will always try and add water, even to a fresh batch, but as a quick rule of thumb – for every 2cm of slump you gain it will reduce the 28 day strength by 3-4 MPa. Mercifully the next load came swiftly and with a lot of needle and form vibrating we avoided forming a cold joint in the pour.
Our concrete supplier have a really handy guide for ordering pre mixed concrete which I would recommend a quick read. Doesn’t mention dags though…
Holdfast Training Services Trial Blog 
Are Holcim charging you for the rejected concrete?